Fluorinated polymers containing sulfonic acid functional groups, due to their ion conducting properties, have found widespread use in the manufacture of electrolyte membranes for electrochemical devices such as electrolysis cells and fuel cells. Notable examples are for instance proton exchange membrane (PEM) fuel cells which employ hydrogen as the fuel and oxygen or air as the oxidant.
Generally, said membranes require excellent ion conductivity, gas barrier properties (to avoid the direct mixing of hydrogen and oxygen), mechanical strength and chemical and thermal stability at the operating conditions of the cell.
During use, the membranes have to withstand three different membrane degradation mechanisms, i.e. chemical, thermal and mechanical degradation. The latter is known to cause early life failures due to perforations, cracks, tears and pinholes.
Several approaches have been developed in the art to improve the mechanical stability of the membranes, with the final aim to increase their durability and lifetime. Typically, these approaches comprise chemical cross-linking, mechanical reinforcement by inorganic fillers and reinforcement with a mechanically stable polymeric matrix. A review of these approaches has been provided by SUBIANTO, S., et al. Physical and chemical modification routes leading to improved mechanical properties of perfluorosulfonic acid membranes for PEM fuel cells. Journal of Power Source. 2013, vol. 233, p. 216-230.
However, while on the one hand the use of inert reinforcements improves the mechanical properties of the membranes, on the other hand it results in a reduction in proton conductivity, with a consequent worsening the electrochemical properties of the membrane.
It has also been disclosed in the art that a sulphonic ionomer can be mixed with a fluoroelastomer.
For example, US 2013/0022894 (GM GLOBAL TECHNOLOGY OPERATIONS, LLC) 24 Jan. 2013, US 2010/0044616 (GM GLOBAL TECHNOLOGY OPERATIONS, INC.) 25 Feb. 2010 and US 2010/047657 (GM GLOBAL TECHNOLOGY OPERATIONS, INC) 25 Feb. 2010 disclose blends of perfluorosulfonic acid ionomers with Kynar® 2751, that is a partially fluorinated copolymer of vinylidene fluoride and hexafluoropropylene.
US 2005/0239912 (SOLVAY SOLEXIS S.P.A.) generally discloses that a ionomer can be mixed with a fluoroelastomer, for example a TFE/perfluoromethylvinyl ether copolymer, in an amount between 0 and 50% by weight with respect to the ionomer. The ionomer and the fluoroelastomer mixture can be physical blend of solid polymers or of polymerization latexes.
However, this application discloses no specific composition comprising a ionomer and a fluoroelastomer, nor provide any detailed indication on how mixing latexes in order to avoid coagulation of ingredients thereby contained.
US 2002/0014405 (AUSIMONT S.P.A.) also generally discloses that a ionomer can be mixed with a fluoroelastomer, for example a TFE/perfluoromethylvinyl ether copolymer, in an amount between 0 and 50% by weight with respect to the ionomer. The ionomer and the fluoroelastomer mixture can be physical blend of solid polymers or of polymerization latexes. In addition, this application exemplifies membranes obtained by physically blending a solid mixture of a ionomer and a perfluoroelastomer copolymer TFE/perfluoromethylvinylether, molding in press the blend to obtain a film and then, acidifying the film to completely transform the —SO2F groups into sulphonic groups —SO3H.
However, the physical blend does not allow forming a mixture wherein the solid ionomer and the solid perfluoroelastomer are homogeneously dispersed. Also, said physical blends have little use from the industrial point of view, as they cannot be used to prepare membranes by casting techniques or by impregnating nano-porous supports.
On the other hand, when blending latexes of the two different polymers, the two latexes are known to show different colloidal behaviour, so that fluoroelastomer latex, more sensitive to coagulation phenomena, may coagulate and separate from the mixture, hence rending mixing totally ineffective.